The preference for CT imaging in conditions previously examined by traditional excretory or intravenous urography (IVU) is now nearly overwhelming, as more studies confirm CT's advantages and address its limitations.
In the past few years, several studies have demonstrated the superiority of nonenhanced helical CT in detecting urinary tract calculi. One of IVU's remaining advantages was its lower x-ray dose. However, more recent studies have shown low-dose CT exams are also equal or superior to IVU in detecting stones.
A study by radiologists at Ontario's McMaster University, reported in the April issue of Radiology, concluded that their 7-mm collimation, 2:1 pitch protocol is "superior to IVU and clinically adequate for diagnosis renal colic." Yet at 2.8 mSv, the radiation dose was three-quarters to half that in earlier reported protocols (Radiology April 2000, Vol. 215:1, pp. 51-54).
Another study reported at the March European Congress of Radiology arrived at a similar conclusion regarding low-dose CT urography.
Dr. V.P. Sukhumar of John Radcliffe Hospital in Oxford presented the results from 50 patients seen in emergency rooms at four U.K. hospitals over seven months. All presented with acute flank pain and suspected obstructing ureteral calculi.
The patients were imaged with low-dose unenhanced spiral CT, including axial images from the top of the kidneys to the symphysis pubis. The slice thickness was 5 mm, and the pitch was 1.5. Each patient underwent a subsequent IVU exam within six hours following the CT scan.
A single radiologist interpreted the CT results without knowledge of the IVU findings, and later performed an unblinded reading that compared the results of the procedures to each other.
Ureteric calculi were found in 44 patients on both unenhanced spiral CT and IVU, while five patients had stones visible only on CT. In one study, CT was unable to differentiate a vesicoureteric junction calculus from a phlebolith, and one patient had a bowel mass presenting as acute flank pain. There were no other significant pathological findings, Sukhumar said.
"CT urography is a cost-effective technique in terms of time and economy compared to IVU, is able to show extraurinary [tract] pathology, and is diagnostically equivalent [to IVU] or better," Sukhumar said. "It is particularly useful in patients with kidney failure or allergies, and when a clinical exam is in doubt, [CT] can within a short time give vital differential diagnostic information."
In addition, he said, CT is more comfortable for patients and comes at a lower cost, and also avoids the risk and expense of contrast agents. On the other hand, he noted, low-dose CT still carries a higher radiation dose than IVU and should perhaps not be used in very young patients or those undergoing repeated examinations.
Sukhumar also said that CT is disadvantaged because it does not show the degree of obstruction by calculi. But researchers from the University of Texas Health Science Center in Houston appear to have substantially addressed this concern in a study published late last year, which found the extent of perinephric edema seen on nonenhanced CT predicted the degree of obstruction in ureterolithiasis patients with 94% accuracy (Radiology, December 1999, Vol. 213:3, pp.663-667).
By Eric Barnes
AuntMinnie.com staff writer
May 1, 2000
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